Enhanced charge-transfer emission in polyimides by cyano-groups doping.

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Enhanced CT Emission in Polyimides by Cyano-Groups Doping Maria Sansebastian, Virginia Martinez-Martinez, Alberto Maceiras, Jose Luis Vilas, Iñigo Lopez-Arbeloa, and Luis M. Leon J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b00845 • Publication Date (Web): 31 Mar 2015 Downloaded from http://pubs.acs.org on April 8, 2015

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The Journal of Physical Chemistry

Enhanced CT Emission in Polyimides by CyanoGroups Doping María San Sebastián†, Virginia Martínez-Martínez‡*, Alberto Maceiras§, José Luis Vilas†, Iñigo López-Arbeloa‡, Luis Manuel León,,†,§ †

Basque Center for Materials, Applications, and Nanostructures (BCMaterials), Parque

Científico y Tecnológico de Bizkaia, Camino de Ibaizabal, Building 500 – 1st. 48160 Derio, Spain ‡

Molecular Spectroscopy Laboratory, Departamento Química Física, Facultad de Ciencia y

Tecnología, Universidad del País Vasco UPV/EHU, P. Box 644, 48080 Bilbao, Spain. §

Macromolecular Chemistry Laboratory, Departamento Química Física, Facultad de Ciencia y

Tecnología, Universidad del País Vasco UPV/EHU, P. Box 644, 48080 Bilbao, Spain

ABSTRACT Alternating different donor-acceptor monomers, conjugated polyimides (PI) are successfully synthesized by chemical imidization through in situ silylation of diamines. A detailed photophysical characterization is performed in the monomers and different polymers in tetrahydrofuran solutions. The emission spectra of the related donor-acceptor polymers with electron withdrawing cyano groups showed broader and more intense fluorescence bands in

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comparison to the polymer without -CN groups. The new emission band results from the contribution of two different charge-transfer pathways: i) An intramolecular CT (ICT) state in the donor monomers due to the presence of strong electron withdrawing CN groups, and ii) a intramolecular CT complex (CTC) in the PI polymer between the donor-acceptor monomers, which is red-shifted and shows longer lifetime respect to the ICT of the diamine monomers. The very wide emission band is a very interesting feature for obtaining white light from UV light.

INTRODUCTION The interest in aromatic polyimides (PIs) has grown enormously over the past decades because of their well-known excellent properties. Aromatic polyimides possess great thermo-oxidative stability, high mechanical strength, and high solvent and radiation stability, which make them useful in the field of high temperature resistant polymers1. Moreover, their properties can be easily modified for specific applications by using different monomers. Nowadays, polyimides are being investigated for applications in advanced technologies2, as insulating or dielectric materials3, sensors (piezoelectric polymers4–7 or magnetoelectric laminates8,9), in the field of energy applications (fuel cells10,11, gas separation membranes12), composites,13,14 and nanomaterials15. Another interesting field is their photophysical properties with many applications16–23

in

photovoltaic,

electrochromic,

photochromic,

thermo-optical,

and

electroluminescent devices. There is a large variety of monomers and several methodologies for polyimide synthesis. However, polyimides often have low solubility in common solvents, thus making their processing either difficult or too expensive to be commercially viable. The most common technique (two-step method) involves the reaction between a dianhydride (acceptor monomer)


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and a diamine (donor monomer) at room temperature in a dipolar aprotic solvent, to yield a soluble poly(amic acid) (PAA) as a precursor. Next, films are casted and thermally dehydrated to produce the final imide form24–29. There are some disadvantages with this process, such as inefficient cyclization, the difficulty in removal of water and the formation of microvoids in the final material. Therefore, there is a great interest in a procedure that would allow toobtain soluble polymers directly in the imide form in a single step. The direct generation of soluble polyimides can be achieved in different ways. The most common method is the one-step polycondensation method30,31, although it requires high temperatures. In an alternative method at ambient conditions, reported by Oishi et al.32,33, silylated diamines are used. The polycondensation reaction involves a nucleophilic substitution at the carbonyl group of the dianhydride with the diamine. Silylated diamines have shown to be more nucleophilic than normal diamines. Poly(amic trimethyl silyl ester) is produced in the first step, which can be converted into polyimide via chemical imidization promoted by acetic anhydride in combination with organic bases as activating agents34–37. However, there is an important disadvantage to this method, regarding water sensitivity and rigorous control of the reaction conditions, but it can be avoided by using in situ silylated diamines, adding trimethylchlorosilane (TMSCl)38. The characteristic properties of polyimides result from the rigid polymeric structures, highly symmetrical polar groups, and strong intermolecular interactions. The strong interactions originate from intra- and inter-chain charge transfer complex (CTC) formation and electronic polarization, hence the interest in studying the photophysical properties of these materials39. Charge transfer structure in polyimides also controls their absorption, photoreactivity, and photoconductivity, making them very promising materials; this is because polymers with


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photonic and optic properties are increasingly needed40,41. CTC is also responsible for the color in polyimides, concurrently with other possible components such as chromophoric units, impurities from starting materials or side reaction products (isoimides)42,43. PIs have strong absorbance in the visible region of the UV-visible spectra and are pale yellow or deep reddish yellow because of their conjugate aromatic structures and/or the inter-molecular CTC formation. Polyimides with a lighter colour are obtained in less favored CTC between the electron-donor (diamine) and the electron-acceptor (dianhydride) moieties44. Regarding the fluorescence derived from the CTC, its efficiency will decrease and gradually displaced to red with the increase of CT ability,

39

. Thus, photophysical properties can be improved via chemical modification;

introducing polar groups in the chain modifies the donor-acceptor behavior, and subsequently CTC formation. Most photophysical studies in aromatic polyimides have been performed in solid-state (films)39. In this work, several soluble polyimides containing electron withdrawing – CN groups were synthesized, being excellent candidates for specific characterization. We analyzed the photophysical properties of the monomers and PIs in solution in function of the number of –CN groups. The aim is to develop novel functional polyimides for optoelectronic devices; the energy exploitation of ultraviolet, an abundant light in environment, is a very interesting approach from a technological perspective 19,20,39,45–48.

EXPERIMENTAL Polyimide synthesis and characterization Polyimides were synthesized via chemical imidization37 by generating silylated diamines in situ38,49, using 4,4’-oxydiphthalic anhydride (ODPA) and different diamines. The following diamines were used: 1,3-bis(3-aminophenoxy)benzene (a monomer with no cyano groups, M0CN), 2,4-di(3-aminophenoxy)benzonitrile (one cyano group in relative position 2,4, M1CN),


4

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and 1,3-bis-2-cyano-3-(3-aminophenoxy)phenoxy-benzene (with two cyano groups, M2CN). All the diamines were synthesized following a previously reported method6, with the exception of M0CN, which was commercially purchased. Also, the 4,4’-oxydiphthalic anhydride (ODPA) was used without further purification. The method of synthesis through the formation of silylated diamines in situ using a tertiary base such as pyridine (Py) as activating agent, is useful in the case of sterically hindered amines and amines with strong electron-withdrawing groups, whose low reactivity could be improved through silylation50. In addition, the salt formation reaction competing with the dianhydride is avoided, the inter-chain crosslinking, making it soluble in most organic solvents, a requisite for the photophysical study we are pursuing. Figure 1 shows the structures of the synthesized polyimides.

O

O O

N

O

N

O

O

O

Polyimide 0CN (P0CN) O

O O

N

O

N

O

O CN

O

Polyimide 2,4 (P1CN) O O N O

N

CN

CN

O O

O

O

O

O

Polyimide 2CN (P2CN)

Figure 1. Repetitive units of the synthetized polyimides.


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Molecular weights (Mn) and polydispersities (I) were determined by gel permeation chromatography (GPC) using a Waters 1515 HPLC isocratic pump and a Waters 2414 refractive index detector, equipped with two TSKgel (Tosoh) chromatographic columns with exclusion limits of 1.5·105 and 1.5·106, respectively. Dimethylformamide was used as eluent at a flow rate of 1 cm3·min-1. Polystyrene or poly(methyl methacrylate) standards were used for calibration. Thermal

behavior

was

studied

by differential

scanning

calorimetry

(DSC)

and

thermogravimetric (TG) analysis. DSC measurements were performed using a METTLER DSC 822e equipment. The synthesized poly(amic acid)s were heated from 30 to 250ºC at a heating rate of 20ºC·min-1, under nitrogen atmosphere, in aluminum pans. The glass transition temperatures (Tg) were determined as the intersection between the extrapolation of the baseline and the tangent line at the inflection point. Thermogravimetric (TG) measurements were performed in a METTLER TGA/SDTA 851e thermobalance. Samples (~10 mg) were heated from room temperature up to 900ºC at 10ºC·min1

under nitrogen atmosphere. The initial degradation temperature was determined from the

intersection between the tangent to the baseline and the inflection point in the thermogram.

Photophysical properties UV-Vis absorption and fluorescence spectra were recorded on a Varian model CARY 4E spectrophotometer and a SPEX Fluorolog 3-22 spectrofluorometer , respectively. The fluorescence spectra were corrected from the from the wavelength dependence of the detector sensibility. Photophysical properties were measured in solutions at a concentration of 10−3 in THF, at room temperature. Emission spectra of more concentrated samples (0.1 M) were


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registered in suspension using 0.1 cm quartz cuvettes and recorded by front face mode (reflection) on the same side of the sample illuminated by detecting the emitted light at 22.5º with respect to the incident beam. Radiative deactivation curves were recorded by means of the time-correlated single-photoncounting technique (Edinburgh Instruments, model FL 920), with a time resolution of 30 ps after the deconvolution of the excitation pulse. The excitation was carried out with diode lasers (PicoQuant, models LDH 370, LDH410, and LDH 440) at 370, 410 nm, and 440 nm, with pulses of 150 ps fwhm (full width at half-maximum), a repetition rate of 20 MHz for windows of 100 ns and 5 MHz for windows of 200 ns, and a power of 0.5 mW. The erratic scattering signal of the laser was avoided in the detection channel by filtering the excitation light with the corresponding cut-off filters. Two types of analyses can be performed from the recorded fluorescent decay curves: -Lifetime analysis: fluorescence decay curves were measured with 10000 counts at the maximum for different emission wavelengths: 450, 470, 500, 550, and 580 nm. The FAST software was used for the global analysis; this software allows the simultaneous analysis of the set of multiexponential decay curves recorded at the different excitation and emission wavelengths:

= exp− ⁄ + exp− ⁄ + (1)

where Ai represent the pre-exponential factors associated with the statistical weights of each exponential and τi are the lifetimes of each exponential decay. The software links the lifetime values τi and varies their statistical weights Ai in the fitting procedure of the all decay curves recorded at the different excitation and emission wavelengths.


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-Time-resolved emission spectroscopy (TRES): fluorescence decay curves were recorded as a function of the emission wavelength in the 415-650 nm range (wavelength increment of 3 nm3 nm) for a fixed recording time (150 s per wavelength). The emission spectra at different times after excitation were obtained by averaging the integrated fluorescence intensity for different time-windows in the 0-100 ns time interval after the excitation pulse.

RESULTS AND DISCUSSION Material characterization Table 1 shows the molecular weights (Mn), polydispersity (I) , glass transition temperature (Tg), and thermal degradation temperature (Td) of the synthesized polyimides through in situ silylated diamines (chemical imidization). The molecular weights and polydispersity of all the chemically imidizated polyimides are of the same order. Glass transition temperature increases with the number of –CN groups, as a consequence of a higher chain rigidity (Figure 2A).

Table 1. Characterization of the synthetized polyimides. Sample

Mn·10-4

I

Tg (ºC)

Td (ºC)

P0CN

2.7

1.3

142.0

520.4

P1CN

4.6

1.2

156.0

493.8

P2CN

3.4

1.5

170.1

485.9

Figure 2B shows the thermograms and initial degradation temperature values corresponding to the main degradation step (Td). In a first degradation step, no more than 4% of mass loss occurs, representing the elimination of the solvents;, while the degradation of polyimides occurs in a second step .


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A

B

100

P0CN P1CN P2CN

P0CN P1CN P2CN

endo

% mass

-1

dQ/dT (W g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40 50

100

150

200

250

100

200

temperature (ºC)

300

400

500

600

700

800

900

temperature (ºC)

Figure 2. A) DSC and B) TGA curves of the in situ chemically imidizated polyimides through the formation of silylated diamines. Photophysical study In order to determine the influence of the electron withdrawing -CN groups on the donor ability of the diamines, quantum simulations were made. In the simulation of the HOMO and LUMO energy levels (Table S1 in Supporting Information), the presence of electron withdrawing -CN groups in the diamines donor monomers progressively reduced their electron donor nature (decreased EHOMO) and enhanced their electron affinity (reduced ELUMO), but the ODPA dianhydride monomer still showed the highest electron acceptor capacity, acting as an acceptor in charge transfer processes along the polymer.

Monomer absorption, excitation, and emission spectra The absorption spectra of all the monomers (Figure 3A) exhibit the typical cut-off profile and do not show any strong absorption bands over 350 nm. However, recording the excitation spectra of the monomers at 400 nm (Figure 3B), narrow bands in the UV region (at around 320-350 nm, Table 2), are revealed, ascribed to the S0-S1 (π -π*) transition. It is worth noting, that the


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excitation spectra of the M1CN and M2CN monomers are broader than that those obtained for ODPA and M0CN, with a shoulder at around 360 and 370 nm, respectively. Regarding the emission fluorescence spectra (Figure 3B), narrow bands were recorded for dianhydride (ODPA) and diamine M0CN, centered at 363 nm and 362 nm, respectively, while diamines M1CN and M2CN showed broader and red-shifted emission bands (430 and 445 nm, respectively), resulting in high Stokes shifts (Table 2). These results indicate that an Intramolecular Charge Transfer (ICT) is activated in M1CN and M2CN diamine monomers due to the presence of the strong electron-withdrawing –CN groups and the electron donating ether and amine groups in the phenyl ring. Although generally ICT states do not present absorption bands because are favored in the excited state after excitation of the LE “Locally Excited” state, M1CN, and particularly M2CN, display an increase in the absorbance at the 350-400 nm range (Figure 3A),with a shoulder at around 360-370 nm in the excitation spectra (Figure 3B, Table 2).

Table 2. Maximum (shoulder) in the excitation (λexc) and fluorescence (λfl) spectra, Full Width at Half Maximum of the fluorescence band (FWHM), and Stokes shifts (∆νst) for the monomers. Monomers λexc (nm)

λfl (nm) FWHM (nm) ∆νst (cm-1)

ODPA

333

363

67

2482

MOCN

320

362

65

3625

M1CN

327(360) 430

155

7325

M2CN

348(370) 445

158

6264

Error ± 1 nm


10

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5

A absorbance

4

ODPA M0CN M1CN M2CN

3 2 1 0

250

fluorescence intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

excitation

350

400

450

500

B

emission

ODPA M0CN M1CN M2CN

300

350

400

450

500

550

600

wavelength (nm)

Figure 3. A) Absorption spectra and B) Height-normalized excitation spectra (recorded at λem=400 nm) and height-normalized emission spectra (after excitation at λex=330 nm), for all monomers: ODPA (solid curves), M0CN (dash-dot curves), M1CN (short-dot curves), and M2CN (dash-curves).

Polyamide absorption, excitation, and emission spectra Although the PI dissolved in THF display a pale colored solution (yellow-brown), no discernible bands were detected in the absorption spectra , showing a typical cut-off profile (data not-shown). Conversely, in the excitation spectra of the polyimides, broad bands were obtained


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(Figure 4). As a general trend, the excitation band shifted to higher energies with number of -CN groups in the polymer. P0CN has a broad band at around 440-475 nm, which can be explained by the transition of Charge Transfer Complex (CTC) formed between the dianhydride ODPA (electron acceptor) and M0CN (electron donor); a less important shoulder is attributed to the Locally Excited “LE” state (S0-S1 transition) of the monomers (diamine M0CN and dianhydride). For this polymer, the excitation band does not change with different emission wavelengths (λem = 500 nm and 570 nm). However, for P1CN the excitation spectra recorded at λem= 500 nm shows a band centered at around 370 nm, which could be attributed to diamine M1CN, primarily to its ICT state (registered as a shoulder in the excitation spectra of M1CN, Table 2 and Figure 3B). A clear widening of the band, with a tail up to 550 nm, was obtained at longer emission wavelengths (λem=570 nm), due to the contribution of the CT complex (CTC) between monomers in the polymer. Similarly, in the case of the excitation spectra for polymer P2CN, the broad blue-shifted excitation band at around 330-370 nm could be ascribed to the S0S1 transition of the monomers (dianhydride and diamine M2CN) and the ICT transition in M2CN (Table 2), while the tail up to 550 nm noticeable at longer emission wavelengths (570 nm), to the CTC formation between the M2CM and ODPA monomers in this polymer.


12

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P2CN

P1CN

P0CN

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

350

400

450

500

550

wavelength

Figure 4. Height-normalized excitation spectra for the three polymer recorded at different emission wavelengths: λem=500 nm (solid curves) and 570 nm (dotted curves). These results indicate that the emission from P0CN is mainly populated by the CTC between the ODPA and M0CN, whereas for P1CN and P2CN the emission at shorter wavelengths (≤ 500 nm) is mainly populated through the “LE” of the monomers; their respective CTCs turn up at higher wavelengths (570 nm). In other words, the CTC is the main responsible for the emission of the P0CN polymer while in P1CN and P2CN, both CT species (ICT and CTC) will compete in the fluorescence spectra and their contributions will depend on the emission wavelength. This can be explained by the presence of the –CN group, which trigger the activation of the ICT state in the diamine monomers (M1CN and M2CN) and at the same time, decrease their relative donor nature in regard to the acceptor ODPA moiety, disfavoring the CTC state in the polymer.


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Figure 5 shows the emission spectra of the three polyimides (P0CN, P1CN, and P2CN), at three representative excitation wavelengths according to the excitation spectra (Figure 4): at 370 nm (direct excitation of the diamine monomers and/or its ICT state, Figure 5A), at 440 nm (direct excitation of CTC in the polymer, Figure 5C), and at 410 nm (both ICT and CTC states are populated, Figure 5B). According to the excitation band (Figure 4), the three polymers present similar intensity at 410 nm, providing thus the best estimation of the relative fluorescence efficiency at this excitation wavelength. The emission bands recorded for the polymers with –CN groups (P1CN and P2CN) were much broader (Full Width at Half Maximum, FWHM = 195 nm and 175 nm for P1CN and P2CN, respectively) than those obtained for P0CN (FWHM = 70 nm). These results are attributed to the contribution of two different CT transitions: i) the ICT state in the diamine monomers (M1CN and M2CN) and ii) the CTC formed between the donor-acceptor monomers in the polymers. Some authors

43,51

consider that the CTC could arise not only between donor-acceptor

monomeric units within the same chain (intra-chain CT complex), but also between donoracceptor monomer units within different chains (inter-chain CT complex). In the case of the polyimides studied in this work, subsequent analyses of the fluorescent emission (Figure S1) indicate that CT complexes occur due to intramolecular interactions (intra-chain CT complex) in the polymer since the shape of the spectra does not depend on the concentration of the solution even at very high values (0.1 M). The simultaneous contribution of these two Charge Transfer species (ICT and CTC) was not so far reported in the literature.


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P1CN;

6

P2CN;

P0CN;

6

2.0x10

B

A

C

6

1.5x10

6

6

1.0x10

5

5.0x10

1.5x10

6

1.0x10

5

5.0x10

0.0

400

450

500

550

600

wavelength (nm)

650

700

450

500

550

600

650

700

500

wavelength (nm)

550

600

650

700


2.0x10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 750

wavelength (nm)

Figure 5. Corrected fluorescence spectra of polyimides at different excitation wavelengths: A) 370 nm, B) 410 nm, and C) 440 nm. The corresponding deconvolution into two Gaussian curves (dot-curves) and the theoretical sum of both (dashed-curve) for P1CN and P2CN (R2 ≥ 0.995) are also plotted. P1CN shows a broad fluorescence spectrum with a maxima placed at 575 nm and a shoulder at around 460 nm, assigned to the emission of an intramolecular CTC (between anhydride and diamine monomers with a –CN group) and the ICT emission in M1CN monomer, respectively. After the deconvolution of the emission spectra into two Gaussian curves (Figure 5, dotted curves), the CTC band, centered at 578 nm, shows an area under de emission curve of around 6-8 times higher than that of the ICT band in the diamine monomer, centered at 448 nm, indicating that the CTC transition is favored over the ICT. However, the broad emission band from P2CN, covered almost the whole visible spectra (400700 nm) due to a more balanced contribution of the fluorescence from the ICT emission band of the diamine monomer and the intramolecular CTC in the polymer, placed at 452 and 560 nm with a relative area under the curves of 1:2, respectively, after the deconvolution of the spectra (dotted lines, Figure 5). A similar emission band was registered at all excitation wavelengths in


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this polymer (Figure 5). The fluorescence intensity registered for P1CN and P2CN after direct excitation of their monomers (at 370 or 410 nm, Figure 5A and B) is around twice higher than that obtained at direct excitation of the CTC (at 440 nm, Figure 5C), indicating that the emission (ICT and CTC) occurs more efficiently via non-radiative energy transfer from local excitation of the monomers in comparison with direct excitation of the CT band. Conversely, the relatively narrower fluorescence band registered in P0CN (70 nm) in comparison with P1CN and P2CN is exclusively attributed to the CTC complex of the P0CN, showing much higher fluorescence intensity when directly excited at its CTC band (440 nm, Figure 5C). Consequently, differences in the contribution of the CTC bands (CTC0 < CTC1 < CTC2) result from the decrease of the donating nature of the diamines when –CN groups are introduced (M0CN > M1CN > M2CN), explaining the gradual reduction of CT complex formation due to the increase in the number of –CN groups. Thus, the fluorescence yield of PI with –CN groups, particularly in P1CN, is higher in comparison to that of P0CN because of a decrease of the donating nature of diamine monomers. To get more insights about the photophysics of the above-mentioned ICT and CTC complexes, fluorescence decay curves were registered at the most representative excitation and emission wavelengths. Mostly, multiexponential decay kinetics was observed and in order to study the effect of emission and/or excitation wavelengths, a global analysis was performed (see experimental section). The decay curves obtained for P0CN at different emission wavelengths (500, 550, and 580 nm) were practically identical (data not shown), corroborating that CTC is the only fluorescence component in this polymer. However, differences were found when the excitation wavelengths


16

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were changed (Table 3, Figure S2A). The decay curve shows a tri-exponential fit at 370 nm excitation that becomes bi-exponential at 410 nm, and finally monoexponential with a lifetime of 7.25 ns at 440 nm (Table 3). From this behavior it is possible to assign the different lifetimes to: the shortest lifetimes. τ1 = 0.2 ns, are only visible at the lowest excitation wavelength (370 nm) and are ascribed to the efficient non radiative energy transfer to the Intra Charge Transfer state in the polymer, the τ2 = 1.2 ns is attributed to the lifetime of monomer M0CN and finally the longest lifetime, τ3 = 7.25 ns, to the CTC, characterized at longer excitation wavelength (440 nm) when CTC band is mainly excited. CT states are usually characterized by a broad red-shifted band and long lifetimes.39

Table 3. Fluorescence lifetimes for the three polymers (P0CN, P1CN, and P2CN) registered at different excitation and emission wavelengths after global analysis.

P0CN

P1CN Varying λexc at λem = 500 nm

τi(ns)

P2CN Varying λem at λexc = 410nm

τ(ns) 370

410

440

0.2

79%

-

-

1.2

11%

25%

7.25

10%

75%

Varying λem at λexc= 410 nm

τ(ns) ≤500 nm

> 500nm

≤500 nm

> 500nm

0.67

80%

60%

0.25

70%

55%

-

3.9

20%

34%

1.9

27%

35%

100

20.6

-

6%

9

3%

5%

Conversely, for P1CN, important changes were not registered with the excitation wavelength (data non-shown) but emission wavelength (Table 3, Figure S2B). For all wavelengths, a multiexponential fit was required. The following lifetimes were obtained after the global analysis: τ1 = 0.67 ns, τ2 = 3.9 ns, and τ3 = 20.6 ns. Because the longest lifetime ( 20.6 ns) was only registered at low energies (> 500 nm), it was assigned to the CTC. Similarly, the shortest


17


lifetime (τ1 = 0.67 ns) was assigned to the quenching process of the monomer M1CN due to the extra deactivation path to the CT state, and finally, τ2= 3.9 ns to the fluorescence deactivation of M1CN (Table 3). Using the same criteria for P2CN, the shortest lifetime (τ1 = 0.25 ns) was assigned to the quenched monomer, τ2 = 1.9 ns to the monomer, and the longest lifetime (τ3= 9 ns) to the CTC. 0-1ns;

normalized fluorescence intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

450

2-4ns;

5-10ns;

500

15-40ns;

550

50-100ns

600

650

wavelength (nm) Figure 6. Time-Resolved Emission Spectra (TRES) of the P1CN polymer registered for the time interval 0-100 ns after the excitation pulse (λexc = 410 nm). Time Resolved Emission Spectra (TRES) performed in P1CN nicely confirm the assignation of polymer lifetimes (Figure 6). For short times after the excitation pulse (≤1 ns), the monomer band, centered at around 450 nm, is mainly observed. However, as the time after the pulse increases, the relative intensity of this band decreases in detriment of the red-shifted CTC band. For the higher times after the laser pulse (> 50 ns), the CTC emission band, placed at 578 nm,


18

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could be purely recorded, which perfectly matches with the position of the CTC previously obtained in the emission spectra after the deconvolution process (Figure 5A and B).

CONCLUSIONS The synthesis of polyimides that are soluble in common solvents has enabled their photophysical study in solution. By adding –CN groups to the electron donor diamine moiety in the polymer, an enhanced and broader fluorescence band has been detected in comparison with the polymer without the –CN groups. A single band is observed for polymer P0CN due to the favored Charge Transfer Complexes between the polymer chain donor parts (diamine) and the acceptor ones (dianhydride), centered at 505 nm. In the case of polymers P1CN and P2CN, whose donor diamine units (M1CN and M2CN) were doped with one and two –CN groups (acceptor groups), respectively, the fluorescence band is much wider and intense as a result of the contribution of two species: the Intramolecular Charge Transfer state in monomers M1CN and M2CN (ICT), and the Charge Transfer Complexes formed in the polymer (CTC). The Charge transfer complexes (CTC) are red-shifted and show longer lifetimes in comparison to the intramolecular CT band of their respective diamine monomers (ICT). Intermolecular CT between the different polymers chains was not observed, even at high concentration. The increase in the fluorescence efficiency together with the widening of emission bands, practically covering the whole Visible spectra (400-700 nm), is a very attractive effect for obtaining white light from UV light.

AUTHOR INFORMATION Corresponding Author


19


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*E-mail: [email protected] ACKNOWLEDGEMENTS The authors would like to thank the financial support from the Basque Government (ACTIMAT project IE10-272, Etortek program; PIEZOPOL project S-PE11UN063, the Saiotek programme, and Ayudas para Grupos de Investigación del Sistema Universitario Vasco Program, IT718-13 and IT339-10). V.M.M. acknowledges the Ramón y Cajal contract with the Spanish Ministry of Economy and Competitiveness, (RYC-2011-09505) .

ASSOCIATED CONTENT Supporting Information. HOMO y LUMO energy levels values for all monomers, heightnormalized fluorescence spectra of P1CN in THF at different concentrations, and fluorescence decay curves of the polymers. This information is available free of charge via the Internet at http://pubs.acs.org.

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GRAPHICAL ABSTRACT (FIGURE) ENHANCED CT EMISSION IN POLYIMIDES BY CYANO-GROUPS DOPING The emission of donor-acceptor conjugated polyimides can be tuned by doping with cyano groups. The presence of –CN groups activates a new Intra Charge Transfer (ICT) in donor monomers. Thus, the fluorescence is a consequence of the emission of this new ICT and the typical Charge Transfer Complex (CTC) between the donor and the acceptor moieties in the polymeric chain, achieving a more intense and broader emission that covers almost the whole Vis spectra under UV excitation light. Emission image. P0CN, P2CN, and P1CN under UV light (370 nm).


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Enhanced photoluminescence in CaMoO4:Eu3+ by Gd3+ co-doping.

Enhanced ionic conductivity in polycrystalline TiO2 by "one-dimensional doping".

Sensitivity of pressure sensors enhanced by doping silver nanowires.

Enhanced paramagnetism of mesoscopic graphdiyne by doping with nitrogen.

Doping-enhanced radiative efficiency enables lasing in unpassivated GaAs nanowires.

Optical antenna enhanced spontaneous emission.

Unusually High Optical Transparency in Hexagonal Nanopatterned Graphene with Enhanced Conductivity by Chemical Doping.

Mo doping-enhanced dye absorption of Bi2Se3 nanoflowers.

Enhanced optical absorption via cation doping hybrid lead iodine perovskites.

Enhanced thermoelectric performance of Cu2CdSnSe4 by Mn doping: experimental and first principles studies.

Band-structure engineering of ZnO by anion-cation co-doping for enhanced photo-electrochemical activity.

Enhanced thermal oxidation stability of reduced graphene oxide by nitrogen doping.

Improving surface-enhanced Raman scattering properties of TiO(2) nanoparticles by metal Co doping.

Structural improvement of CaFe₂O₄ by metal doping toward enhanced cathodic photocurrent.

Dynamics enhanced by HCl doping triggers full Pauling entropy release at the ice XII-XIV transition.

Enhanced thermoelectric properties of AgSbTe2 obtained by controlling heterophases with Ce doping.

Core-shell nanostructured "black" rutile titania as excellent catalyst for hydrogen production enhanced by sulfur doping.

Enhanced UV upconversion emission using plasmonic nanocavities.

Enhanced spontaneous emission inside hyperbolic metamaterials.

Enhanced field emission of WS₂ nanotubes.

Color tuning and white light emission via in situ doping of luminescent lanthanide metal-organic frameworks.

Enhancing the emission intensity and decreasing the full widths at half maximum of Ba3Si6O12N2:Eu(2+) by Mg(2+) doping.

Tunable white light emission from single-phased Li2 SrSiO4 :Dy(3+) phosphors by co-doping with Eu(3+).

Enhanced ∼2 μm Emission of Tm3+ in Lu2O3 by Addition of a Trace Amount of Er3.